This invention relates to processes for the production of electrophoretic displays.
The term “electro-optic”, as applied to a material or a display, is used herein in its conventional meaning in the imaging art to refer to a material having first and second display states differing in at least one optical property, the material being changed from its first to its second display state by application of an electric field to the material. Although the optical property is typically color perceptible to the human eye, it may be another optical property, such as optical transmission, reflectance, luminescence or, in the case of displays intended for machine reading, pseudo-color in the sense of a change in reflectance of electromagnetic wavelengths outside the visible range.
Electrophoretic displays have been the subject of intense research and development for a number of years. Such displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays. (The terms “bistable” and “bistability” are used herein in their conventional meaning in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property, and such that after any given element has been driven, by means of an addressing pulse of finite duration, to assume either its first or second display state, after the addressing pulse has terminated, that state will persist for at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of the display element.) Nevertheless, problems with the long-term image quality of these displays have prevented their widespread usage. For example, particles that make up electrophoretic displays tend to settle, resulting in inadequate service-life for these displays.
Numerous patents and applications assigned to or in the names of the Massachusetts Institute of Technology (MIT) and E Ink Corporation have recently been published describing encapsulated electrophoretic media. Such encapsulated media comprise numerous small capsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles suspended in a liquid suspending medium, and a capsule wall surrounding the internal phase. Typically, the capsules are themselves held within a polymeric binder to form a coherent layer positioned between two electrodes. Encapsulated media of this type are described, for example, in U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584; 6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773; 6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,249,721; 6,252,564; 6,262,706; 6,262,833; 6,300,932; 6,312,304; 6,312,971; 6,323,989; 6,327,072; 6,376,828; 6,377,387; 6,392,785; 6,392,786; 6,413,790; 6,422,687; 6,445,374; 6,445,489; 6,459,418; 6,473,072; 6,480,182; 6,498,114; 6,504,524; 6,506,438; 6,512,354; 6,515,649; 6,518,949; 6,521,489; 6,531,997; 6,535,197; 6,538,801; 6,545,291; 6,580,545; 6,639,578; 6,652,075; 6,657,772; 6,664,944; 6,680,725; 6,683,333; and 6,704,133; and U.S. Patent Applications Publication Nos. 2002/0019081; 2002/0021270; 2002/0053900; 2002/0060321; 2002/0063661; 2002/0063677; 2002/0090980; 2002/0106847; 2002/0113770; 2002/0130832; 2002/0131147; 2002/0145792; 2002/0171910; 2002/0180687; 2002/0180688; 2002/0185378; 2003/0011560; 2003/0011868; 2003/0020844; 2003/0025855; 2003/0034949; 2003/0038755; 2003/0053189; 2003/0096113; 2003/0102858; 2003/0132908; 2003/0137521; 2003/0137717; 2003/0151702; 2003/0189749; 2003/0214695; 2003/0214697; 2003/0222315; 2004/0008398; 2004/0012839; 2004/0014265; and 2004/0027327; and International Applications Publication Nos. WO 99/67678; WO 00/05704; WO 00/38000; WO 00/38001; WO 00/36560; WO 00/67110; WO 00/67327; WO 01/07961; WO 01/08241; WO 03/092077; and WO 03/107,315.
An encapsulated electrophoretic display typically does not suffer from the clustering and settling failure mode of traditional electrophoretic devices and provides further advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates. (Use of the word “printing” is intended to include all forms of printing and coating, including, but without limitation: pre-metered coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating; roll coating such as knife over roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; ink jet printing processes; and other similar techniques.) Thus, the resulting display can be flexible. Further, because the display medium can be printed (using a variety of methods), the display itself can be made inexpensively.
One major reason why encapsulated electrophoretic displays can be produced inexpensively by printing processes is that the electrophoretic medium itself has substantial mechanical strength and cohesion; typically the individual capsules are bound together by a polymeric binder to increase the cohesion of the layer. Thus, not only can the display medium itself be printed, but as described in U.S. Pat. No. 6,177,921, an electrode may be formed by printing a conductive material directly on to the electrophoretic medium; alternatively, an electrode pre-formed on a substrate can be laminated on to the electrophoretic medium, which is able to withstand the heat and pressure required for such lamination without damage. In such printed or laminated structures, the mechanical strength and cohesion of the electrophoretic medium maintain the requisite spacing between the electrodes disposed on either side of the medium without any need for mechanical spacers or similar devices to control this spacing. Accordingly, if the electrodes (and any substrates attached thereto) are flexible, the encapsulated electrophoretic display can be curved or rolled without affecting the display qualities of the device; see, for example, Drzaic et al., A Printed and Rollable Bistable Electronic Display SID (Society for Information Display) 98 Digest, page 1131 (1998), which illustrates a flexible encapsulated electrophoretic display being rolled around a pencil without damage.
Furthermore, because of the mechanical strength and cohesion of the electrophoretic medium, such a medium can in principle be applied to any substrate on which an electrode can be provided; for example, the substrate could have an arbitrary three-dimensional shape, as opposed to an essentially laminar sheet which is curved in one dimension. Techniques such as sputtering may be used to apply electrodes to arbitrary three-dimensional shapes, but prior art techniques for applying an electrophoretic medium to such arbitrary shapes leave a great deal to be desired, especially given the need for careful control of the deposition of such a medium to produce optimum optical performance.
Display performance (e.g., its optical performance) and visual appeal (i.e., minimizing visual defects) depends critically on obtaining a high quality coating, that is coatings are preferably of uniform thickness (often a monolayer of capsules is desirable), and contain a high areal density of capsules with a minimum of defects. For example, regions where capsules are not in contact with the electrode or where the surface density of capsules varies laterally with respect to the substrate, or where the coating thickness varies, show up as a degraded dark or white states, non-uniformity in the optical state or graininess, or as non-uniformities during switching respectively.
Some of the printing/coating techniques described above can produce high quality printings/coatings of capsules on to planar or flexible substrates; during coating, flexible substrates are usually constrained so that at least one of the radii of curvature of the substrate is infinite, i.e., the substrate is held in a cylindrical form with the axis of the cylinder perpendicular to the direction of coating. In particular, certain of the aforementioned E Ink and MIT patents and applications describe the use of a metered slot coating technique to produce monolayer capsule coatings and lamination adhesive coatings suitable for use in commercial products.
However, as already mentioned these prior art techniques are not satisfactory for forming, on arbitrary three-dimensional shapes, electrophoretic medium coatings with a sufficiently uniform thickness to give optimum optical performance. While coating methods such as dip or spray coating can be applied to arbitrary three-dimensional shapes, it is difficult to or impossible to achieve uniform capsule monolayers over the substrate surface using these coating techniques.
Other problems encountered with slot coating techniques include:
chatter-like streaks parallel to the coating head (for example, due to vibrations in the coating apparatus); these streaks are believed to result from periodic bunching or jamming of capsules;
streaking in the direction of coating (i.e., perpendicular to the slot of the coating head), believed to be due to capsule jamming or non-uniform flows in delivery of capsules to the coating head;
less than desirable capsule contact (or wetting) with the optical face due to inadequate settling or deformability of the small capsules (of the order of 20-200 μm) typically used in encapsulated electrophoretic displays; and
non-uniformities in coating thickness due to formation of multiple layers of capsules (see the aforementioned 2003/0137717 for a discussion of the advantages of forming only a single layer of capsules on a substrate).
The presence of these types of defects can adversely affect the appearance and optical performance of the display.
Also, as is well known to those skilled in slot coating technology, slot coating imposes limitations upon the viscosity and other physical properties of the material being coated, and typically requires the addition of slot coating additives to control the rheology of the coated material to ensure that the coating does not flow and develop non-uniformities in thickness prior to drying. As discussed in many of the aforementioned E Ink and MIT patents and applications, typically the capsules are mixed with a polymeric binder prior to coating, this polymeric binder serving to form the capsules into a coherent layer after coating and drying. The capsule/binder mixture is then coated on to a polymeric film substrate bearing a conductive coating of indium tin oxide (ITO) or a conductive polymer and dried to form a coherent layer on the conductive-coating bearing surface of the substrate; the opposed surface of the substrate forms the viewing surface of the final display. Although the binder serves several useful functions, including ensuring adequate adhesion of the capsule film to the substrate on which it is coated, excessive amounts of binder can hinder capsule contact with the electrode which is normally present on the substrate, and may also hinder the desirable flattening of the faces of the capsules in contact with the conductive coating (see, for example, the aforementioned U.S. Pat. No. 6,067,185). Because the binder typically has a substantial effect on the viscosity and other physical properties of the capsule/binder mixture, at least in some cases it may be difficult to reduce the amount of binder used and still maintain these physical properties at values compatible with slot coating.
Also, some of the aforementioned E Ink and MIT patents and applications (see especially 2002/0113770) describe displays in which more than one type of capsule is used, the plurality of types of capsules being arranged in a predetermined pattern on a substrate. For example, a full color display could make use of three different types of capsules, say white/red, white/green and white/blue arranged in stripes of triads; such a display could achieve full color without requiring a color filter of the type used in full color liquid crystal displays. However, while conventional printing techniques might be used to prepare large displays of this type having resolutions of (say) less than 10 lines per inch (approximately 0.4 lines per mm), producing high resolution displays of this type with resolutions of about 100 lines per inch (approximately 4 lines per mm) with such conventional techniques is very difficult. Again, while spray or ink jet coating might be used to apply the patterned coatings of capsules, producing monolayer capsule coatings using these methods will be difficult or impossible.
The present invention seeks to provide processes for the production of electrophoretic displays, and in particular for processes for depositing capsules on a substrate, which reduce or eliminate the problems of the prior art processes for depositing capsules described above.